Process for manufacture of novel, inexpensive radio frequency identification devices

ABSTRACT

A novel process for fabricating low cost RFID devices in which a pattern of metallic toner is printed on a substrate and the contacts on a silicon die are placed directly on contact points printed as part of the pattern of metallic toner; the whole device is then heated to both cure the metallic toner into metallic conductors and bond the silicon die to the metallic conductors. Alternatively, the silicon die can be physically attached to the substrate and the electrical pathway between the silicon die and the metallic conductors is established via a transformer coupling comprised of a coil winding on the silicon die and a pattern of coils printed as part of the metallic toner pattern. The pattern of coils can be comprised of individually printed coil loops printed on, and separated by, dielectric layers.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the priority of U.S. Provisional PatentApplication Ser. No. 60/255,490 filed Dec. 15, 2000, the entire contentsand subject matter of which is hereby incorporated in total byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention describes methods for the manufacture of inexpensiveradio frequency identification devices (RFID) that are also very thin incross section so that they can be laminated into paper, tags or labelswithout mechanical interference nor surface distortion.

2. Description of the Related Art

Radio frequency identification and tracking devices (RFID) have shown arapid growth in both function and capabilities. RFIDs are currently usedin everything from anti theft tags, to smart wireless cards toidentification tags for merchandise and many other uses are in thedesign/system specification stage. Examples of currently availablesystems which use RFID tags for merchandise include Tag-It (TexasInstruments) and “iCode” (by Philips Electronics). With a potential needfor billions of such devices, low cost per “tag” and maximumfunctionality are the goals in the market place.

Current manufacturing utilizes photolithographic methods which arecostly, time consuming and can be environmentally hazardous.

While researchers are describing ways to “print” organic transistors ornano-particle inorganic transistors for RFID; their performance levelsare not up to the speeds required for high frequency radio frequencydevices. RFID band-width assignments are expected to be in the 800 to950 MHz range. Therefore standard silicon chips with very highperformance (which are capable of functioning in the desired frequencyrange) and at relatively little cost need to be mounted and electricallyconnected to an inexpensive printed wiring structure. Current methodsfor mounting the silicon chips, such as “flip-chip” methodology, requireequipment for precise alignment of the chip and can also be timeconsuming.

SUMMARY OF THE INVENTION

This invention relates to a process for the manufacture of inexpensiveRFID devices that are mechanically, very thin in dimension and areinexpensive to manufacture because of unique techniques used to producethe metallic wiring structure and to interconnect the silicon devices tothe metallic wiring structure. A metallic toner is printed on thesubstrate in the desired pattern. A thin silicon wafer is placed activeside down on the unsintered metal toner printed pattern, then the wholestructure is heated to a temperature suitable for the substrate (forexample, for a PET substrate 125° C. for approximately 2 minutes)sintering the metal toner and bonding the metal to the electrode pads onthe silicon chip.

In an alternate method of connecting the chip to the substrate, the chipitself contains on its top, active surface a coil of printed metal thatserves as the primary of an air core transformer. The chip ismechanically bonded by a suitable adhesive, in close proximity to asecondary transformer winding printed on the printed wiring structure ofthe “tag” device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an electrical schematic of a preferred embodiment of theinvention.

FIG. 2 shows a schematic of an alternate embodiment of the invention.

FIG. 3 shows the wiring layout of a preferred embodiment.

FIG. 4 shows a cross section of the embodiment of FIG. 3.

FIG. 5 shows the wiring pattern for an alternate embodiment of theinvention with a transformer coil.

FIG. 6 shows a detail of the multi layer patterns of the transformercoil of FIG. 5.

FIG. 7 shows a cross section of an alternate embodiment with atransformer coupling.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a schematic of a preferred embodiment of the invention. Asilicon chip 10 is connected by two pads to loop antenna 12 printed onthe tag substrate. FIG. 3 shows a layout of the “tag” made by theprocess of the alternate embodiment. The loop antenna consists of two ormore turns of metal pattern 20 ending in two pads 22 across which ismounted a Si chip 26 active side down (i.e., the bonding pads on thechip touch pads 22). FIG. 4 shows a cross section of a preferredembodiment of the invention. Substrate 30 is the mechanical carrier orsupport. It preferably is not metal as this would raise losses in thereception and transmission of r.f. energy. Typical substrates that areinexpensive are PET film, PEN film, paper, glass epoxy and the like. IfPET is used, an anti-static layer can be used to enhance theelectrostatic transfer of metal toner to its surface. If paper is used,an adhesion layer is preferably used to fill the pores and fibercavities of the paper and provide adhesion for the metallic tonerparticles to the substrate. In either case the adhesion layer preferablyincludes a resin to promote low temperature processing of the silvertoner into a solid metal conductor. A typical and preferred resin isselected from the DOW chemical series of Saran™ resins though otherresins have worked well.

On top of the anti-stat/adhesion layer, conductor patterns are printedby means of electrostatic printing of metal toners on the anti-statsurface. Typical metal toners include copper, silver, aluminum and gold,with silver being a preferred toner. After drying of the liquid tonerhydrocarbon diluent, the metal toner is sintered by heating to atemperature compatible with the upper temperature limit of thesubstrate. In one embodiment, after drying the toner, the silicon chip26 is placed on the dried powder silver toner, bonding pads down ontothe silver toner pattern. Now the entire assembly is sintered wherebythe silver particles sinter into a solid mass and sinter themselves tothe bonding pads of the chip. Thus the metal traces are sintered and thesilicon chip is bonded to the pads in a single step. This achieves asignificant cost advantage over other production methods.

Finally a liquid resin encapsulation layer, 28, is applied to act as avapor and oxygen barrier. The layer can be applied by various means;spray, liquid roll, silk screening, etc. and cured appropriately tocomplete the final product. Preferred resins include Saran® and epoxyresins.

In summary, the manufacturing steps are:

-   -   1. Print pattern of metal toner;    -   2. Dry diluent from/off of toner;    -   3. Mechanically place silicon chip/die;    -   4. Sinter the structure;    -   5. Overcoat or encapsulate with liquid resin;    -   6. Drying or cross-linking of the overcoat resin.

FIG. 2 illustrates a tag utilizing the transformer coupling aspect ofthe invention. In the device of FIG. 5 a typical 4 turn antenna loop 50having two end points 54, 56 is printed on the edges of the “tag”. Aclear dielectric cross over layer 52 is placed over the section of thetag where the end points 54, 56 are located. This allows for subsequentlayer of patterned metal toner to be printed on the cross-over layerwithout making electrical contact with the underlying toner pattern 50.The area of the dielectric layer above the end points 54, 56 is eitherremoved or is not placed with the rest of the dielectric layer, toenable an electrical connection to the end points 54, 56. Now a secondlayer of metal 58 in the form of one or more loops having end pointslocated directly above, and so connected to end points 54 and 56 isplaced on the dielectric layer thereby completing the circuit andforming a winding for an air core transformer. In summary, the threelayers; a first layer of metal 50, dielectric layer 52, and top metallayer 58 make an electrically continuous loop consisting of a large areaantenna, 50, 28 and a transformer winding, 58, 26.

Additional dielectric layers and metal layers can be added to form multilayered circuits.

FIG. 6 shows the 2nd layer metal co-located over a segment of the 1 stlayer metal to form the coil. To complete the transformer coupling withthe silicon chip the chip contains a output transformer coil 24 and ismounted directly above the coil 50, 28, 58, 26 on the substrate. Whilethe location of the chip is not as critical as when mounting andphysically and electrically connecting the chip to the metal tonercircuit, it is preferred, to increase efficiency of signal/powertransfer, to place the chip as close to the substrate coil as possible,for example, within the locations X-X, 60 and Y-Y 62.

FIG. 7 shows a cross section of a transformer coupling embodiment.Substrate 30 has an antistat/adhesion layer 32 and printed thereon afirst metal layer 70, and a dielectric cross over layer 72. A secondmetal layer 74 completes the circuit as shown in FIG. 5. In a preferredembodiment the first metal layer includes both the antenna loops and anadditional transformer lop. The second metal layer includes one or moretransformer loop which, when connected to the transformer loop on thefirst metal layer, forms a transformer coil have two or more loops.Adhesive layer 76 is placed on the second metal layer 74 and bonds chip78 in close proximity to the transformer winding 58, 26. The thicknessof the adhesive layer 76, typically about 5 microns or less, is smallcompared to the area (x-x, 60; and y-y, 62), of the primary transformercoil which is preferably of the order of about 250×250 microns or more.This assures efficient transfer of energy from the antenna to the chipand from the chip out to the antenna.

Encapsultating layer 28 protects the device from the environment and mayalso have a planarizing effect on the entire structure of the device.

In another embodiment, a substrate with an etched metal pattern iscoated selectively with an adhesive by means of ink jet, ink pen, ortoner like material. The material is a metal filled vinyl, epoxy oracrylic type resin. The conductive material is placed on the electrodesof the metal patterns. A semi-conducting die is placed, electrode sidedown on the conductive pads to make contact to the electrodes of themetal “antenna” pattern. Heating of the structure; substrate, adhesive,and semi-conducting die bonds the die and makes electrical contactbetween “antenna” terminals and die electrodes.

Substrate 90 with etched metal pattern 92, has imaged on its electrodepads, conductive adhesive dots 94. Over this die 96 is accurately placedso that the electrodes on die 96, not shown, align with pads 94. Heatingto achieve re-flow or setting of adhesive 94 is applied as necessary.

Note: adhesive exists in which simple pressure activation is all that isrequired to achieve the bonding step. This is typical of the Eastman910™ type of cyno-acrylic adhesives (i.e. the Crazy Glues). In this casethe die would be pressed on to the adhesive dots to complete the bondingstep, rather than a thermal re-flow step. In some applications thermalre-flow might be undesired as it causes an uncontrolled shrinkage of thesubstrate film (like PET where 1/2% is normally expected). Thisshrinkage negates any degree of overlay accuracy.

EXAMPLES

The examples described below indicate how the individual constituents ofthe preferred compositions and the conditions for applying them functionto provide the desired result. The examples will serve to further typifythe nature of this invention, but should not be construed as alimitation in the scope thereof which scope is defined solely in theappended claims.

Example I

A 25 micron thick PET film was coated with Saran® resin #F-276 (DOW) toa nominal thickness of 1 micron. Parmod Silver Toner E-43 (Parelec LLC,Rocky Hill, N.J.) was mixed to 1.5% by weight concentration to aconductivity of 5 pico siemens per cm. This toner was then imaged on astandard Electrox electrostatic printing plate (Dynachem #5038 dry filmetch resist, exposed to a level of 250 mj/cm²). The silver toner imagewas transferred to the Saran coated PET film. The toner mage was driedat about 40° C.

Next a silicon chip thinned to 10 microns by means practiced by VirginiaSemiconductor Inc. of Richmond, Va. was placed, active side down ontothe silver toner image. The assembly of silicon chip on toner image oncoated PET film was heated to 125° C. for two minutes. Good conductivityof the silver was achieved with excellent bonding of the chip to thesilver.

Example II

A three layer substrate was prepared using the same techniques ofExample 1. A Saran coated PET film was imaged with Parmod toner andthermally cured into a useful conductive pattern. A dielectric “crossover” pattern of a Saran toner was printed and reflowed into a pin holefree layer. Note, the electrode pads of the conductive pattern of thefirst layer are left uncovered by the Saran cross-over layer. A secondmetal layer was printed on the Saran layer interconnecting theelectrodes.

A portion of the pattern of the first layer and the pattern of thesecond metal layer were configured to form a coil pattern (“secondarywinding”).

A dot of thermally or pressure activated adhesive was applied to the“secondary winding” region of the substrate and a silicon die with a“primary winding” contained on its surface was accurately placed on thisadhesive. Bonding is completed by heat or pressure.

Example III

A film substrate like 50 micron PET film coated with 500 Angstroms ofpure aluminum metal was imaged in an Indigo NV Omnius Webstream printer.The Indigo toner was printed directly on the aluminum metal. Thealuminum film printed with toner was then etched in a mild caustic bathremoving the unprotected metal. The dried substrate was then stripped ofthe toner in the electrode areas with toluene.

A conductive adhesive (AbleStick#862B) was applied in small dots to thealuminum electrodes. A silicon die (Micro Chip Technologies of PhoenixAriz., #MC-355) was placed, face down, on the conductive adhesive dotpattern; both boding the chip and making useful electrical to thesubstrate metal pattern.

Example IV

The devices of Examples I, II and III were spray coated with Saran resin(#F-276, DOW) and then heated to cure the resin and form a protectivecoating on the entire device.

1. An RFID device comprising a substrate; an antenna means on saidsubstrate, said antenna means being comprised of a metal toner printedin a pattern comprising at least one loop: at least one silicon chip;and a connection means for electrically connecting said antenna meansand said silicon chip.
 2. The RFID device of claim 1 wherein saidconnection means is comprised of an electrically conductive adhesive. 3.The RFID device of claim 1 wherein said connection means is comprisedof: a first coil means connected to said antenna means and a second coilmeans connected to said silicon chip. wherein said first coil means andsaid second coil means are proximally located thereby facilitatingelectrical communication.
 4. The RFID device of claim 3 wherein saidfirst coil means is comprised of at least two loops wherein each of saidat least two loops is separated by a layer of dielectric.
 5. The RFIDdevice of claim 4 wherein said first coil means has at least a first anda second loop each loop having two endpoints, wherein a first loop islocated on said substrate and a second loop is located on a dielectriclayer located above said first loop, wherein one endpoint of said firstloop is connected to said antenna means and the second endpoint of saidfirst loop is connected to the first endpoint of said second loopthrough a hole in the dielectric layer and wherein the second endpointof said second loop is connected to said antenna means through anopening in the dielectric layer.
 6. The RFID device of claim 3 whereinsaid first coil means is comprised of at least two loops wherein each ofsaid at least two loops is separated by a layer of dielectric.
 7. TheRFID device of claim 3 wherein said second coil means is located on saidsilicon chip.
 8. The RFID device of claim 1 wherein said antenna meansis printed on said substrate.
 9. The RFID device of claim 8 whereinprinting is by electrostatic or inkjet printing methods.
 10. The RFIDdevice of claim 3 wherein said connection means is printed byelectrostatic or inkjet printing methods.
 11. The RFID device of claim 7wherein said second coil means is printed by electrostatic or inkjetprinting methods.
 12. The RFID device of claim 1 further comprising aprotective coating.
 13. A process for the manufacture of RFID devicesconsisting of the following: a. Electrostatic printing of a metal toneron a coated substrate, said printing comprising an antenna having atleast one loop; b. The drying of this metal toner image; c. Themechanical placement of a silicon die on this dried, printed metal tonerimage; d. The heating of this assembly to a suitable temperature causinga sintering of the metal toner particles together and a sintering ofthem to the electrode pads of the silicon die; and e. The overcoat ofthe die/substrate with a protective coat.
 14. The process of claim 13 inwhich the metal toner is made of silver.
 15. The process of claim 13 inwhich the substrate is PET film or paper.
 16. The process of claim 13 inwhich the substrate is coated with an adhesion/sintering layer thatpromotes both sintering of the metal particles and their adhesion to thesubstrates.
 17. The process of claim 16 in which this coating is chosenfrom Saran™ resins of Dow Chemical.
 18. A process for the manufacture ofrf-ID devices in which: a. metal toner is printed on a suitablesubstrate in a suitable pattern; b. the pattern in the area of siliconchip mounting is configured into a single or multui-turnelectro-magnetic coil; c. this pattern is suitably processed into aconductive metal pattern; d. the substrate is coated with a suitableadhesive layer; e. a silicon die possessing an electromagnetic coilpattern of metal around its periphery is placed and aligned to the metaltoner coil pattern of the substrate; and f. the bonding reaction betweendie and adhesive coated substrate is completed by suitable means. 19.The process of claim 18 in which the die has been “thinned” to a valuebelow 50 microns.
 20. The process of claim 18 where the substratethickness is less than 50 microns.
 21. The process of claim 18 where theoverall thickness of the final part is between 10 and 100 microns.